Perturbative calculations ignore the Bose enhancement of the amplitude due to the presence of identical states. Depending on the coupling, this enhancement can be very significant and lead
to resonant production [1]}, [2]}, [3]}. Such a regime can be described semiclassically by analyzing the equations of motion for the DM field \(s\) .
In what follows, we compare the corresponding
resonant particle production via
operators \( {\cal O}_3\)  and \({\cal O}_4 \) .
